Nick is one cynical little cookie. Even though Nick reserves explicit judgment on the characters, Fitzgerald still manages to implicitly criticize through his narrator's tone. (Think about how ludicrous Myrtle seems when, although she isn't upper class, she still tries to look down on her husband.) The characters are sometimes slighted by the ironic tone, and we the readers are forced to read with the same cynicism that Fitzgerald writes.
Let’s take a look at two passages. This first one is from Chapter 1, when Nick is hanging out with the Buchanans and Jordan for the first time:
"I love to see you at my table, Nick. You remind me of a – of a rose, an absolute rose. Doesn’t he?" She turned to Miss Baker for confirmation: "An absolute rose?"
This was untrue. I am not even faintly like a rose. She was only extemporizing, but a stirring warmth flowed from her, as if her heart was trying to come out to you concealed in one of those breathless, thrilling words. Then suddenly she threw her napkin on the table and excused herself and went into the house.
We see here that Nick is all too aware of the ridiculousness of certain social circumstances; he’s also aware of the seductive quality of the upper class, even though he feels it’s somewhat empty. He narrates this scene as though his personal recollection is the objectively true version, Isn't there is room for alternate interpretations? While reading, we took Nick’s account of the story as the truth, but there probably is a way of viewing the story’s events without his cynical assumptions.
Nick also has a good grip on what he thinks is righteous or reproachable, and he hands that to his audience as the absolute true judgment of a person or an act. For instance, take a look at this excerpt from the last few pages of the novel, when Nick has become disillusioned with his former acquaintances:
I couldn’t forgive [Tom] or like him, but I saw that what he had done was, to him, entirely justified. It was all very careless and confused. They were careless people, Tom and Daisy – they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made....
I shook hands with him; it seemed silly not to, for I felt suddenly as though I were talking to a child.
How would you feel about Tom and Daisy if Nick’s tone were less cynical – if his version of the story was completely objective? Would it be possible to empathize with them?